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Original question:

BQ78350-R1: Supply current and SOC and Remaining Capacity

BQ78350-R1: Self-Discharge and Supply current (revisited)

JPAlleyne
Expert 1565 points
Part Number: BQ78350-R1
Other Parts Discussed in Thread: BQ78350

Hi,

I brought up an issue I had a while ago where I asked how to incorporate the supply current of the AFE and Gauge (18mA) into battery discharge in order to preserve the accuracy of the gas gauge. The reply stated that I could incorporate it into the self-discharge of the battery and that it should be negligible. Maybe my math is wrong but 18mA does not seem to be a negligible value. We are using an 11000mAH battery. At 18mA the battery will drain in about 25 days. The Lithium ion is about 0.1% a day self-discharge which should be 1000 days to fully self-discharge. Am I doing something incorrectly in this calculation? It seems as if the electronics will incur a 4% discharge everyday. Even though it's negligible for a discharge cycle, it isn't negligible for storage. How can I deal with this issue.

Thanks,

JP

  • 0
    TI__Prodigy 680 points
    Hi,
    Our team is currently enjoying the Thanksgiving break and will come back to you when we return, during the week of November 26th.
  • 0 in reply to bdefar
    TI__Guru* 81034 points
    Hi JP,

    The supply current (18mA) sounds extremely high for the AFE and the gauge. Why is the current this high? The Electronics Load compensation for the BQ78350 only goes up to 765 uA, so I understand why you might use the Self Discharge parameter as another way to account for this. Normally the supply current of the AFE and gauge are very small and will not account for a significant discharge, but I agree 18 mA is not negligible.

    There is a Shutdown mode to reduce power consumption to a minimum. This is commonly used for shipping or long term storage.

    Best regards,
    Matt
  • 0 in reply to Matt Sunna
    Expert 1565 points

    Hi Matt,

    I've seen around the same current draw (around 15 mA) on the eval board as well.Do you think there is something on that circuit that draws all that excess current? This is even when the LEDs are off. Even in shutdown mode I see that amount of current being drawn.

    Thanks,

    JP

  • 0 in reply to JPAlleyne
    TI__Guru* 81034 points
    Hi JP,

    The EVM will draw current in this range across the resistor divider, but the AFE and gauge ICs will not draw that much current. Is there something else on your board that might account for the 18mA?

    Thanks,
    Matt
  • 0 in reply to Matt Sunna
    Expert 1565 points
    Which resistor divider? I realized that we never tested the current drawn from the battery while the balance leads are connected. We have only done it from a power supply. I will do a current test to see how much current is drawn with the battery plugged in.
  • 0 in reply to JPAlleyne
    TI__Guru* 81034 points
    The resistor divider that simulates the battery cells.
  • 0 in reply to Matt Sunna
    Expert 1565 points

    Okay, that's what I thought you meant. Right now I have just the balance leads connected and I'm testing the current running from the top cell through to the board. I'm disregarding batt+ (since the top cell and batt+ are electrically connected) and I'm reading a peak of 2.5 mA in normal mode. I'm assuming at certain points (during balancing when the FETs turn on) there will also have current running from the balance leads right?

    In sleep mode I get 1.75 mA and the 2.5 mA (I guess on the periodic wake-up)

    In shutdown mode I get 6.8 uA

    Does this sound about right? Is this an accurate of making the current draw measurement?

    thanks Matt,

    JP

  • 0 in reply to JPAlleyne
    TI__Guru* 81034 points
    Hi JP,

    That still seems a little high compared to the datasheet parameters. I can look at it on an EVM on Monday - there may be something else pulling a little current.

    Regards,
    Matt
  • 0 in reply to Matt Sunna
    Expert 1565 points
    Hi Matt,
    Anything will help. I also have the high side driver on my circuit.
  • 0 in reply to JPAlleyne
    TI__Guru* 81034 points
    JP,

    The high side driver should draw a little more current, but I think you should still be much lower. Is there anything else on your board that might be drawing some current (LEDs, smaller size pull-up resistors, resistor dividers, etc)?

    Matt
  • 0 in reply to Matt Sunna
    Expert 1565 points

    Thanks, Matt. I'll review my circuitry and see if I have any low value resistors in there.

    Thanks,

    JP